Mediation of exciton concentration on magnetic field effects in NPB : Alq3-based heterojunction OLEDs

Organic light-emitting diodes (OLEDs) are considered one of the most promising new display technologies owing to their advantages, such as all-solid-state, high color gamut, and wide viewing angle. However, in terms of special fields, the brightness, lifetime, and stability of the devices need further improvement. Therefore, heterojunction devices with different concentrations were prepared to regulate device brightness. The brightness of the bulk heterojunction device is enhanced by 9740 cd m−2, with a growth rate of about 26.8%. The impact of various temperatures and various exciton concentrations on the device magneto-conductance (MC) and magneto-electroluminescence (MEL) was investigated. Experimental results demonstrate that the exciton concentration inside the device can be tuned to improve optoelectronic properties and organic magnetic effects. The complex spin mixing process inside the bulk heterojunction device is deeply investigated, which provides a reliable basis for the design of bulk heterojunction devices.


Introduction
In the past three decades, various properties of OLEDs have attracted considerable attention from researchers in all sorts of aspects, such as operating voltage, brightness, and response speed. [1][2][3][4][5] In 1987, Tang and VanSlyke prepared organic lightemitting diodes (OLEDs) with low driving voltage using this (8-hydroxy-quinoline)aluminum (Alq 3 ) as the light-emitting material. 6 In 2003, Kalinowski prepared OLEDs using materials without any magnetic properties and found magnetic-eld effects (MFEs) in the devices. 7 In 2010, Lee controlled the magneto-conductance (MC) response of a poly(3hexylthiophene) (P3HT)-based photovoltaic device by applied magnetic eld, bias voltage, built-in potential, and interfacial dipole layers. 8 MFEs include the magneto-conductive effect and the magneto-electroluminescence (MEL) effect. MC is the change of current with the magnetic eld, and MEL is the change of luminous intensity with the magnetic eld. 8,9 In the subsequent studies, researchers began to focus on the inuences of external voltage, ambient temperature, functional layer doping, and other factors on the amplitude and line shape of MC and MEL curves. [10][11][12] MC and MEL curves are the "ngerprints" of the internal microscopic processes of OLEDs, which visually reect the role of carriers and provide a method for researchers to study different spin-mixing processes inside the devices.
Aer decades of research, OLEDs have undergone rapid development and signicant improvements in their optoelectronic properties, leading to broad applications in panel displays and lighting applications. [13][14][15] However, achieving a high standard of full-color gamut requires further improvement in core indexes, such as brightness, current efficiency, lifetime, and the stability of the devices. 16,17 In 2015, Xiang et al. found in planar heterojunction and bulk heterojunction devices that there was a large difference in linearity between the two at 100 K. 18 The MEL of planar heterojunction devices decreased because triplet exciplex gathered near the planar heterojunction, prompting the triplet-triplet annihilation (TTA), while the TTA could not occur because there were few triplet exciplexes around the bulk heterojunction. Then Chen et al. prepared bulk heterojunction devices of rubrene and C 60 with MEL curves showing paradoxical voltage dependence, and the analysis discovered that the EL of the half-bandgap voltage devices was not due to the formation of single excitons, originating from TTA in rubrene lms. 19 Recently, Yuan et al. improved the electroluminescence performance of exciplex by doping organic spacers into the emitter of bulk heterojunction exciplex; this method achieved low driving voltage, high efficiency, and inefficient attenuation. 20 To date, most of the studies were focused on the bulk heterojunction solar cells, which exhibit low cost and high energy efficiency, and how to apply their advantages to OLEDs is worth further exploration. 21,22 In this study, heterojunction OLEDs are prepared by vacuum evaporation, using a P-type semiconductor material N,N ′di(naphthalene-1-yl)-N,N ′ -diphenyl-benzidine (NPB) [reference to the chemical structure in Fig. 1(a)] and an N-type semiconductor material tris(8-hydroxyquinoline)aluminum (Alq 3 ) [reference to the chemical structure in Fig. 1(a)]. Fig. 1(b) is the schematic diagram of the device structure energy level diagram of ITO/MoO 3 /NPB/NPB : Alq 3 /Alq 3 /CsCl/Al. We also prepared ITO/MoO 3 /NPB/Alq 3 /CsCl/Al in contrast to the heterojunction devices.

Results and discussion
In the solid state, there are two primary modes of contact between the two organic semiconductor materials: interlayer contact, which results in a planar heterojunction with minimal material overlap; and intermixing, which leads to an intrinsic heterojunction with signicantly greater material overlap than a planar heterojunction.
The materials constituting planar PN heterojunctions mainly include two categories of P-type and N-type materials. The requirements for P-type materials include: (1) having a suitable highest occupied molecular orbital (HOMO) to facilitate hole injection from the anode into the P-type material; and (2) a suitable hole mobility to enable hole transport from the anode interface to the heterojunction interface. The requirements for N-type materials include: (1) a suitable minimum unoccupied molecular orbital (LUMO) for electron injection from the cathode into the N-type material; and (2) a suitable electron mobility for electron transport from the cathode interface to the heterojunction interface. NPB, as a P-type semiconductor material, has a HOMO energy level of 5.4 eV and a high hole mobility (3 × 10 −4 cm 2 V −1 s −1 ). Alq 3 , as an Ntype semiconductor material, has a LUMO energy level of 3.1 eV and a high electron mobility (11.4 × 10 −6 cm 2 V −1 s −1 ). Therefore, the contact between NPB and Alq 3 results in the formation of planar PN heterojunction and intrinsic PN heterojunction. [38][39][40] The voltage-brightness and voltage-current density curves are shown in Fig. 1(c). Heterojunction devices with NPB : Alq 3 = 1 : 1 have higher luminance than comparison devices, indicating that the heterojunction can improve the brightness. The maximum luminance of 1 : 1 is 46 040 cd m −2 , when the voltage is 9 V. However, the maximum reduction drops sharply when the doping ratio changes from 1 : 1 to 1 : 4, and decreases to 17 210 cd m −2 . The electron mobility of Alq 3 in the device is 11.4 × 10 −6 cm 2 V −1 s −1 , and the hole mobility of NPB is 3 × 10 −4 cm 2 V −1 s −1 , so the hole carriers in the devices are the majority carriers. In theory, as the guest Alq 3 doping concentration increases, the hole concentration in the emitting layer (EML) decreases, electron concentration increases, electron and hole combination probability increases, and device luminance increases. However, in the experiment, as the guest Alq 3 doping concentration increases, the luminance of the device does not increase rather it decreases. To investigate this interesting phenomenon, we measured the MEL curves of the devices and analyze the energy transfer of the devices. As shown in Fig. 4(b), as the Alq 3 doping concentration increases, the reduced energy transfer process [Förster energy transfer (FRET) and Dexter energy transfer (DET)] leads to a decrease in the number of singlet excitons (S 1,Alq3 ), and thus the luminescence of the device is diminished. Meanwhile, the decrease in the number of triplet excitons (T 1,Alq3 ) leads to more S 1,Alq3 excitons being converted to T 1,Alq3 excitons through the singlet ssion (SF) process (S 1,Alq3 + S 0 / T 1,Alq3 + T 1,Alq3 ), which further reduces the luminance of the device. In addition, the increase of Alq 3 doping concentration makes the Alq 3 molecular spacing decrease, which promotes the SF process in the device. In summary, the device maximum luminance decreases as the Alq 3 doping concentration increases. In the following text, this is analyzed more specically.
At the same voltage, the current density of the four devices decreases when the doping ratio changes from 1 : 1 to 1 : 4. For example, at 9 V, the current density of devices with NPB : Alq 3 = 1 : 1, 1 : 2, 1 : 3, and 1 : 4 are about 632 mA cm −2 , 166 mA cm −2 , 65 mA cm −2 , and 35 mA cm −2 , respectively. Inside the devices, electrons and holes form a compound current I in the organic layer, and the total current I of the device can be expressed as: 42 where I 0 e and I 0 h are leakage currents of electron and hole in the devices, respectively, and I e and I h are currents I of electron and hole in devices, respectively. The electron mobility of Alq 3 is much smaller than the hole mobility of NPB. Therefore, at the same voltage, as the Alq 3 doping concentration increases, the electron current I e in the device increases slightly and the hole current I h decreases signicantly, resulting in a lower total device current and making the current density of the device lower. Fig. 1(d) shows the normalized electroluminescence spectra of the devices at room temperature under a bias voltage of 7.5 V, with the luminescence peak of 520 nm for the 1 : 1 device and 528 nm for the 1 : 2, 1 : 3, and 1 : 4 devices. It indicates that the energy of the doped main body NPB is completely transferred to the guest Alq 3 , NPB does not contribute to the luminescence, and the exciplex is not generated in the devices. With the growth of Alq 3 doping concentration, the luminescence peak has a slight red shi, which is attributed to the aggregation of Alq 3 molecules in the luminescent layer. 23 In considering the impact of external voltage on the magnetic effect of heterojunction devices, Fig. 2 shows the MEL curved line of four devices at different voltages. The denition of MEL is where EL(B) and EL(0) represent the brightness of the device with and without an external magnetic eld, respectively. From Fig. 2, it can be seen that the MEL curves all show the same trend of change. With Alq 3 concentration increases, the MEL linearity of the four devices also did not change, that is, the magnetic eld is 0-25 mT when the MEL curve increases rapidly, and the magnetic eld is 25-330 mT when the MEL curve increases slowly.
To better understand the micromechanical evolution of MEL reection in devices, the microscopic mechanism of the device is shown in Fig. 4. In organic electroluminescent devices, holes and electrons can be compounded in the NPB : Alq 3 layer to form polaron pairs. Since there are two spin directions of electrons and holes, there are also two types of polaron pairs, namely, singlet polaron pair (PP S ) and triplet polaron pair (PP T ). 10 Under the hyperne interaction (HFI), 24,25 PPs and PP T will undergo spin-mixing and transform into each other (PP S 4 PP T ), which is called inter-system crossing (ISC) induced by HFI [ Fig. 4(a)]. 26,27 When the magnetic eld is applied, PP T becomes PP T+ , PP T0 , and PP T− by Zeeman splitting; however, the power of the PP T0 state is similar to PPs and is converted to one another (PP T0 4 PP S ), this suppresses PP S / PP T , causing an increase in the amount of PP S . 8 This inhibition will reach saturation within a few mT. 24,28 The Coulomb interaction leads to the further conversion of PP S and PP T into singlet exciton (S 1,Alq3 ) and triplet exciton (T 1,Alq3 ). S 1,Alq3 is capable of directly radiating prompt uorescence (PF). At room temperature, T 1,Alq3 can not radiate luminescence directly due to the forbidden transition.
For NPB : Alq 3 , it can be observed from Fig. 1(b) that the highest occupied molecular orbital (HOMO) of the doped guest Alq 3 is higher than that of NPB, and its lowest unoccupied molecular orbital (LUMO) is higher than the LUMO energy level of NPB, so no carrier trap is formed. In addition, the Förster energy transfer (FRET) and Dexter energy transfer (DET) processes are very obvious in host-guest doped devices. FRET can be achieved at distances of a few nanometers due to the need for charge Coulomb interaction, which occurs mainly between the host and guest S 1,Alq3 excitons. For FRET, the rate constant (K FRET ) is proportional to the overlapping integral of the PL spectrum of the host material NPB and the absorption spectrum of the guest material Alq 3 . According to the literature, 12,41 the overlapping area of the absorption spectrum of guest Alq 3 and the PL spectrum of host NPB suggests that FRET can effectively occur in NPB : Alq 3 devices. The DET between host and guest T 1,Alq3 excitons can be achieved at a distance of a few angstroms by hopping electron and hole transport between molecules. The triplet state energy level of the host material NPB (E(T 1, NPB) = 2.32 eV) is higher than that of the guest material Alq 3 (E(T 1,Alq3 ) = 2.05 eV), 12 and the Dexter energy transfer (DET) between the host and the guest can be effectively generated in devices with NPB : Alq 3 . As shown in Fig. 4, the formed PP S and PP T consist of two parts, one is formed on the host material and the other is formed on the guest material. S 1,Alq3 and T 1,Alq3 on the host material are formed on the guest material by FRET and DET, respectively. 29 Fig. 2 shows that at the low magnetic eld the MEL curve rises rapidly because magnetic eld B inhibits the ISC process of HFI generation, which increases the S 1,Alq3 concentration and the luminescence brightness. As the magnetic eld continues to increase, the inhibitory effect reaches saturation and the MEL slowly rises and gradually saturates in the range of 25-300 mT. Fig. 2(a) shows that the amplitude of MEL rise with increasing voltage for NPB : Alq 3 = 1 : 1 (whose magnetic eld is the absolute value of 0-25 mT) decreases continuously from 5.2% at 3 V to 2.9% at 8 V. When an external magnetic eld is applied, the electrons and holes are dynamically adjusted to the concentration of S 1,Alq3 and T 1,Alq3 by feeding at specic angular u = m B gB/ħ frequencies, where m B denotes the Bohr magnetic moment, g denotes the Lund factor, ħ denotes the approximate Planck constant, and B denotes the magnetic eld. With increasing voltage and thus current density, leading to an increase in the concentration of triplet excitons and a decrease in the concentration of singlet excitons, there is an increase in voltage and a decrease in MEL amplitude instead. In addition, the MC curves of the four devices at different voltages were also studied, as shown in Fig. 3. The denition of MC is where I(B) and I(0) represent the currents of the devices with and without an external magnetic eld, respectively. The spinmixing process of exciton inside the device can be analyzed by MC. Due to the stronger dissociation of PP S , it is signicantly more likely than PP T to dissociate into free charges and thus enhance the conduction current of the device. This means that the magnetic eld suppresses the ISC effect, which contributes to an increase in the amount of PP S and secondary carriers formed by its dissociation, resulting in enhanced MC within a smaller magnetic eld range. Similarly, the suppression of the ISC process by the applied magnetic eld quickly saturates. 6 So the MC of Fig. 3 will show a rapid rise from 0 to 25 mT and a slow rise and gradual saturation from 25 to 330 mT.
To investigate the inuence of exciton concentration on heterojunction devices, the MEL and MC of devices with different Alq 3 dosages at the same voltage (5 V, for example) are compared, as shown in Fig. 5(a) and (b). MEL and MC in the gure show similar linearity to that in Fig. 2, a fast rise caused by HFI and ISC at low magnetic elds, soon saturated under the high eld. However, from Fig. 5(a) and (b) we can see that the amplitude of the MC and MEL curves increase with growing Alq 3 dosages at 5 V, and both MC and MEL increase by 1.4%, but all are smaller than the MC and MEL amplitude of NPB : Fig. 4 (a) PP S and PP T change process and (b) the schematic of energy transfer and microscopic process in device. Alq 3 = 1 : 4. The hole mobility of NPB is much larger than the electron mobility of Alq 3 ; therefore, the charge is unbalanced. It can be seen from Fig. 1(b) that holes are more easily injected than electrons. When NPB : Alq 3 = 1 : 1, the amount of NPB in the light-emitting layer increases, and too many holes make the device unbalanced; when NPB : Alq 3 = 1 : 4, the amount of Alq 3 in the light-emitting layer increases, and the growth of electrons leads to an increase in the degree of equilibrium of the device. Therefore, from NPB : Alq 3 = 1 : 1 to NPB : Alq 3 = 1 : 4, it shows the process of the device from disequilibrium to equilibrium. As shown in Fig. 4(b), the singlet ssion (SF) process (S 1,Alq3 + S 0 / T 1,Alq3 + T 1,Alq3 ) dominates within the Alq 3 molecule. As the doping ratio of Alq 3 increases, resulting in increased SF processes in the luminescent layer, and the phenomenon of increasing amplitude of MC and MEL appears. At the same time, the SF process is enhanced and more S 1,Alq3 excitons in the luminescent layer are converted to T 1,Alq3 excitons, reducing the prompt uorescence of the devices. Thus, it can be seen that the exciton dosage has a momentous inuence on the organic magnetic effect. According to the spin-statistics principle, the ratio of PP S to PP T based on spin pairing within organic electroluminescent devices is 1 : 3. 30,31 In uorescent devices, S 1,Alq3 generates uorescence, while T 1 does not contribute directly to the luminescence due to the forbidden transition. It was found that under certain conditions, S 1,Alq3 and T 1,Alq3 can be interconverted, and two T 1,Alq3 can produce an S 1,Alq3 and a groundstate molecule, a process known as TTA, 32-35 creating delayed uorescence. However, the external magnetic eld will have an inhibitory effect on the TTA, weakening the delayed uorescence and causing the electroluminescence intensity to how a reduced trend in the high magnetic eld. The TTA process and the T 1,Alq3 exciton lifetime can be expressed by the following equation.
where I d denotes the initial intensity of the TTA process, s denotes the T 1,Alq3 exciton lifetime, and b denotes the constant associated with the pulse width. When the temperature is higher, the T 1,Alq3 lifetime is shorter due to the effect of thermal noise, such as phonons, and the TTA process is difficult to occur in the high magnetic eld part. When the temperature drops from 145 K to 45 K, the lifetime of T 1,Alq3 is extended and the TTA process dominates at this time, but the external magnetic eld will inhibit the TTA effect, so the MEL curve at 45 K and 9 K shows a slow decrease in the high magnetic eld part, 36,37 and the temperature is lower, the fall in the high magnetic eld part of the MEL is more obvious. In conclusion, the temperature can effectively affect the spin-mixing process inside the device.

Experimental
Each functional layer material was vaporized on the substrate glass using JD-450 coating equipment, and the functional layer material was presented in the form of a thin lm on the substrate glass. Heterojunction devices with the structure of ITO/MoO 3 (5 nm)/NPB (30 nm)/NPB : Alq 3 (1 : x, 70 nm)/Alq 3 (40 nm)/CsCl (0.6 nm)/Al (120 nm) [x = 1, 2, 3, and 4] were prepared. Before the experiment, ITO substrate glass needs to be preprocessed as follows: rst, the ITO surface is scrubbed for 2-3 min to remove impurities, such as oil and dust, from the ITO surface, and the substrate glass is rinsed repeatedly aer scrubbing until there is no detergent. Then ITO surface is ultrasonically cleaned with acetone for 30 min, followed by wiping for 5-6 min. Aer wiping, ITO is ultrasonicated with deionized water, anhydrous ethanol, and acetone for 15 min, dried for 5 min, and ozonated for 30 min. The pretreatment of the ITO substrate can improve surface function. In the experiment, the treated ITO substrate glass was put into the coating equipment, placed the material in the vacuum chamber, and then various functional layer materials were vaporized in turn. During evaporation, the vacuum level was kept at about 10 −4 Pa to prevent the organic materials from being oxidized by water.
In the experiments, a lm thickness detector (SI-TM206C) was used to monitor the evaporation rate and lm thickness. The device optoelectronic data and magnetic effect data were measured by Janis CCS-350S and LakeShore-643, respectively. A PR-655 spectrometer was used to measure the electroluminescence spectra of the devices.

Conclusions
In summary, four heterojunction devices with the structures of ITO/MoO 3 (5 nm)/NPB (30 nm)/NPB : Alq 3 (1 : x, 70 nm)/Alq 3 (40 nm)/CsCl (0.6 nm)/Al (120 nm) [x = 1, 2, 3, and 4] were prepared. Compared with the reference device, the maximum brightness of the heterojunction device is 46 040 cd m −2 , which is an improvement of 9740 cd m −2 . It was found that the MC and MEL curve amplitudes decreased with increasing applied bias voltage for the same doping ratio at room temperature and increased with growing Alq 3 doping concentration for the alike bias voltage. In addition, the impact of temperature on the MEL curve of the device was researched. At 45 K and 9 K, the MEL curves show a different phenomenon from room temperature, the high magnetic eld part decreases slowly, which is caused by the TTA that plays a dominant role at low temperature. In this study, the inuence of exciton concentration on the magnetic effect of heterojunction device is investigated in depth, and the complex spin mixing process inside the heterojunction device is explored and to provide a data reference for the design of heterojunction devices.

Conflicts of interest
There are no conicts to declare.